Android: allow building Vulkan/XNNPACK backends as separate shared libraries

extension/android/CMakeLists.txt

@@ -116,24 +116,69 @@ if(TARGET qnn_executorch_backend)
 endif()
 
 if(TARGET xnnpack_backend)
-  executorch_target_link_options_shared_lib(xnnpack_backend)
-  list(
-    APPEND
-    link_libraries
+  set(_xnnpack_so_link_libs
     xnnpack_backend
     XNNPACK
     pthreadpool
     cpuinfo
     xnnpack-microkernels-prod
   )
   if(TARGET kleidiai)
-    list(APPEND link_libraries kleidiai)
+    list(APPEND _xnnpack_so_link_libs kleidiai)
+  endif()
+
+  if(EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED)
+    # Same rationale as EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED below: ship
+    # xnnpack_backend as its own libxnnpack_executorch_backend.so so the
+    # consumer can opt out at install time. Link only the backend itself
+    # (whole-archived so the register_backend static initializer fires) and
+    # the XNNPACK third-party libs + executorch_core. Kernel registration
+    # archives stay in libexecutorch_jni.so so loading both shared libraries
+    # together does not trigger Error::RegistrationAlreadyRegistered (0x16).
+    add_library(xnnpack_executorch_backend SHARED
+      ${EXECUTORCH_ROOT}/extension/android/jni/jni_noop.cpp
+    )
+    executorch_target_link_options_shared_lib(xnnpack_backend)
+    target_link_options(xnnpack_executorch_backend PRIVATE -fopenmp -static-openmp)
+    target_link_options_gc_sections(xnnpack_executorch_backend)
+    target_link_libraries(xnnpack_executorch_backend
+      PRIVATE ${_xnnpack_so_link_libs} executorch_core log
+    )
+    set_target_properties(
+      xnnpack_executorch_backend PROPERTIES LINK_FLAGS "-Wl,-rpath='$ORIGIN'"
+    )
+    # executorch_jni still needs cpuinfo headers / lib for thread count detection.
+    list(APPEND link_libraries cpuinfo)
+  else()
+    executorch_target_link_options_shared_lib(xnnpack_backend)
+    list(APPEND link_libraries ${_xnnpack_so_link_libs})
   endif()
 endif()
 
 if(TARGET vulkan_backend)
-  executorch_target_link_options_shared_lib(vulkan_backend)
-  list(APPEND link_libraries vulkan_backend)
+  if(EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED)
+    # Build vulkan_backend as a standalone shared library that React Native
+    # Executorch can ship as an opt-in artifact (libvulkan_executorch_backend.so).
+    # The .so links only vulkan_backend (with --whole-archive so its
+    # register_backend static initializer fires) plus executorch_core/vulkan_schema.
+    # Kernel registration libs (optimized_native_cpu_ops_lib, custom_ops, etc.)
+    # stay in libexecutorch_jni.so to avoid duplicate Kernel registration when
+    # both .so are loaded together (Error::RegistrationAlreadyRegistered, 0x16).
+    add_library(vulkan_executorch_backend SHARED
+      ${EXECUTORCH_ROOT}/extension/android/jni/jni_noop.cpp
+    )
+    executorch_target_link_options_shared_lib(vulkan_backend)
+    target_link_options_gc_sections(vulkan_executorch_backend)
+    target_link_libraries(vulkan_executorch_backend
+      PRIVATE vulkan_backend vulkan_schema executorch_core log
+    )
+    set_target_properties(
+      vulkan_executorch_backend PROPERTIES LINK_FLAGS "-Wl,-rpath='$ORIGIN'"
+    )
+  else()
+    executorch_target_link_options_shared_lib(vulkan_backend)
+    list(APPEND link_libraries vulkan_backend)
+  endif()
 endif()
 
 if(EXECUTORCH_BUILD_KERNELS_LLM)

extension/android/jni/jni_noop.cpp

@@ -0,0 +1,3 @@
+// Empty translation unit so SHARED libraries that only need to whole-archive
+// other static archives (e.g. libvulkan_executorch_backend.so) have at least
+// one source file for CMake to invoke the linker on.

extension/llm/custom_ops/CMakeLists.txt

@@ -67,7 +67,11 @@ endif()
 
 list(TRANSFORM _custom_ops__srcs PREPEND "${EXECUTORCH_ROOT}/")
 
-if(NOT EXECUTORCH_BUILD_XNNPACK)
+if(NOT EXECUTORCH_BUILD_XNNPACK OR EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED)
+  # custom_ops doesn't actually call into xnnpack_backend (no XNNPACK refs in
+  # its sources). The transitive PUBLIC link is what was previously dragging
+  # XNNCompiler/XNNExecutor into libexecutorch_jni.so via the WHOLE_ARCHIVE on
+  # custom_ops, which defeats EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED. Drop it.
   list(APPEND custom_ops_libs extension_threadpool)
 else()
   list(APPEND custom_ops_libs extension_threadpool xnnpack_backend)

scripts/build_android_library.sh

@@ -33,6 +33,8 @@ build_android_native_library() {
   fi
 
   EXECUTORCH_BUILD_VULKAN="${EXECUTORCH_BUILD_VULKAN:-OFF}"
+  EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED="${EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED:-OFF}"
+  EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED="${EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED:-OFF}"
   XNNPACK_ENABLE_ARM_SME2="${XNNPACK_ENABLE_ARM_SME2:-ON}"
 
   cmake . -DCMAKE_INSTALL_PREFIX="${CMAKE_OUT}" \
@@ -52,6 +54,8 @@ build_android_native_library() {
     -DEXECUTORCH_BUILD_QNN="${EXECUTORCH_BUILD_QNN}" \
     -DQNN_SDK_ROOT="${QNN_SDK_ROOT}" \
     -DEXECUTORCH_BUILD_VULKAN="${EXECUTORCH_BUILD_VULKAN}" \
+    -DEXECUTORCH_BUILD_VULKAN_BACKEND_SHARED="${EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED}" \
+    -DEXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED="${EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED}" \
     -DXNNPACK_ENABLE_ARM_SME2="${XNNPACK_ENABLE_ARM_SME2}" \
     -DFLATCC_ALLOW_WERROR=OFF \
     -DSUPPORT_REGEX_LOOKAHEAD=ON \
@@ -68,7 +72,18 @@ build_android_native_library() {
   # Copy artifacts to ABI specific directory
   local SO_STAGE_DIR="cmake-out-android-so/${ANDROID_ABI}"
   mkdir -p ${SO_STAGE_DIR}
-  cp "${CMAKE_OUT}"/extension/android/*.so "${SO_STAGE_DIR}/libexecutorch.so"
+  cp "${CMAKE_OUT}"/extension/android/libexecutorch_jni.so "${SO_STAGE_DIR}/libexecutorch.so"
+
+  # Copy standalone Vulkan backend shared library if built. Used by React
+  # Native Executorch as an opt-in artifact when the app enables Vulkan.
+  if [ -f "${CMAKE_OUT}"/extension/android/libvulkan_executorch_backend.so ]; then
+    cp "${CMAKE_OUT}"/extension/android/libvulkan_executorch_backend.so "${SO_STAGE_DIR}/"
+  fi
+
+  # Same for XNNPACK as a standalone shared library.
+  if [ -f "${CMAKE_OUT}"/extension/android/libxnnpack_executorch_backend.so ]; then
+    cp "${CMAKE_OUT}"/extension/android/libxnnpack_executorch_backend.so "${SO_STAGE_DIR}/"
+  fi
 
   # Copy QNN related so library
   if [ -n "$QNN_SDK_ROOT" ] && [ "$ANDROID_ABI" == "arm64-v8a" ]; then